It looks like the author tries to make a point, cherry picking articles that fit his view.
First, there is no causal relationship, something he mentions "more research is needed to determine if multitasking is physically damaging the brain (versus existing brain damage that predisposes people to multitask)", but dismisses it right after "it's clear that multitasking has negative effects".
It is also interesting how he went from "smaller density" to "damage", which implies some kind of destruction rather than adaptation. Then there is all that part about EQ, which mixes bad behavior (not paying attention), with other forms of multitasking like having plenty of stuff open on your computer. I mean, I can monotask during a meeting by not paying attention at all. Then it connects it to the "brain damage" study by saying that some of EQ-related parts of the brain are also related to multitasking, without showing the whole picture.
The IQ and performance related part is more convincing, but also isn't really tied to the "brain damage" part. It is also a bit confusing as evidenced by a commenter asking "what is multitasking".
That IQ drops when you are doing two things at once is kind of obvious, but it is nice to have a study to back it up. The question is "is it worth it"? And now comes what I think is the only insightful part: media multitasking, basically having more than one thing on your screen(s) makes it harder to switch tasks properly.